• Three conclusions: fiber is not necessary for a healthy diet, low-carb diets can still include gut-irritating foods, and microbiome manipulation for weight loss lacks enough support.

Fiber and constipation

• Fiber is the carbohydrate portion of plant food that humans cannot digest; soluble fiber ferments in the colon into gases and short-chain fatty acids, while insoluble fiber mainly bulks stool.
• Conventional guidance links fiber with bowel cancer prevention, diverticulosis prevention, hemorrhoid relief, blood-sugar control, and constipation relief, so patients worry when cereals and breads are removed.
• In the constipation study, 63 patients used high-fiber, reduced-fiber, or zero-fiber diets; the zero-fiber diet removed vegetables, cereals, fruit, wholemeal bread, and rice.
• High-fiber intake increased symptom prevalence, reduced fiber lowered symptoms modestly, and zero fiber left no patient with constipation, straining, bloating, bleeding, or pain.
• Every zero-fiber participant had one bowel action per day, while the high-fiber group averaged one bowel action every 6.83 days.
• Functional constipation criteria concern stool passage through the anal sphincter, so enlarging stool with bran makes the exit problem harder.
• Fiber does not moisten stool; stool moisture remains stable across different fiber and water intakes.
• Soluble fiber reaches the colon, ferments, and makes hydrogen and other gases, so bloating and pain can occur in a gastrointestinal tract with limited volume.

Low-carb diet implications

• Low-carb diets remove many cereals and breads, but common substitutions such as berries, cauliflower, and almonds can still be high in fiber.
• Large intakes of nuts or cauliflower mash can therefore cause gas, bloating, or discomfort.

Short-chain fatty acids and ketones

• Short-chain fatty acids are produced when colonic bacteria ferment fiber, salvage energy, and can feed colonocytes.
• Colonocytes can use short-chain fatty acids after conversion to ketones, and circulating ketones from nutritional ketosis can reach colonocytes directly through the blood.
• The IBD comparison favors circulating ketones over short-chain-fatty-acid enemas for colonic inflammation.
• The animal-based diet graph had higher short-chain-fatty-acid production than the plant-based diet graph, so fiber is not unique as a route to these fatty acids.

Fiber and diabetes

• Extra fiber can reduce glucose and insulin spikes when carbohydrate intake is high, as in the ADA diet comparison.
• Carbohydrate restriction gives fewer carbohydrates and less blood-glucose rise to blunt.
• The high-fiber diabetes graph still rose above 10 mmol/L, while the LoBAG low-biologically-available-glucose diet gave much flatter blood-glucose control.

Human gut design and FODMAPs

• Human anatomy lacks the large cecum of herbivores, so humans are not built to ferment fiber on a herbivore scale.
• FODMAPs are poorly absorbed short-chain carbohydrates that reach the colon, ferment into gas, and draw water through osmotic activity.
• FODMAP effects overlap with IBS symptoms, including watery stools, bloating, pain, and constipation.
• Methane from bacterial metabolism slows transit and can contribute to constipation.
• Low-FODMAP diets improve symptoms in about three quarters of IBS cases across repeated studies.
• In the non-celiac gluten sensitivity bar study, fructan caused worse symptoms than gluten or a control bar, making fructan a stronger explanation for those symptoms than gluten.
• Low-carb diets avoid much wheat, but FODMAPs still occur in foods such as cauliflower.
• Polyols, the sugar alcohols in many low-carb bars and sweets, are poorly absorbed, draw water, make gas, and can cause loose stools or diarrhea.

Microbiome and weight

• The old 10-to-1 bacteria-to-human-cell ratio is exaggerated; the revised estimate is about 1.3 bacteria cells for each human cell.
• More than 1,000 bacterial types live in the gut, but most belong to Firmicutes or Bacteroidetes.
• Firmicutes have been associated with obesity, Bacteroidetes with weight loss, and Bacteroidetes rise during weight loss in the graph used here.
• Gut bacteria change quickly because each species grows under specific nutritional and environmental conditions.
• Diet can shift gut bacteria within a single day, and high-fat ketogenic diets in children with epilepsy have been linked with Bacteroidetes.
• The unresolved question is whether bacteria drive weight loss or diet drives both weight loss and bacterial change.

Obese microbiota mouse experiment

• In the 2006 mouse experiment, germ-free mice receiving microbiota from obese mice gained more fat than those receiving microbiota from lean mice.
• The mechanism in the graph was greater energy harvest through short-chain-fatty-acid production from fiber.
• This does not mean transplanting bacteria into a normal human colon will cause weight loss, because germ-free mice lack existing bacterial competition.
• The extra harvested energy depended on fiber, so the result would not apply the same way to a low-fiber diet.
• Short-chain fatty acids become ketones, and ketones can suppress appetite, so free-eating animals could eat less after producing more ketones.
• Diet change permits weight loss and gut bacteria change alongside it.
• The key issue is causation versus correlation.

Trehalose and C. difficile

• Trehalose is a two-glucose sugar added to the food supply more recently and used to lower the freezing point of foods such as ice cream.
• C. difficile can use trehalose, and higher trehalose intake after the early 2000s was linked with more pseudomembranous colitis associated with that organism.
• Diet changes gut bacteria, but changing gut bacteria for weight loss remains beyond current understanding.

Final conclusions

• Fiber is not necessary for a healthy diet.
• Even low-carbohydrate diets can include high-fiber foods or FODMAPs that cause stomach upset.
• Microbiome manipulation for weight loss is currently too speculative.

References

• [00:02] Stopping or reducing dietary fiber intake reduces constipation and its associated symptoms — https://doi.org/10.3748/wjg.v18.i33.4593
• [00:10] Diet rapidly and reproducibly alters the human gut microbiome — https://doi.org/10.1038/nature12820
• [00:10] Beneficial effects of high dietary fiber intake in patients with type 2 diabetes mellitus — https://doi.org/10.1056/NEJM200005113421903
• [00:11] Effect of a high-protein, low-carbohydrate diet on blood glucose control in people with type 2 diabetes — https://doi.org/10.2337/diabetes.53.9.2375
• [00:13] Methane, a gas produced by enteric bacteria, slows intestinal transit and augments small intestinal contractile activity — https://doi.org/10.1152/ajpgi.00574.2004
• [00:13] A diet low in FODMAPs reduces symptoms of irritable bowel syndrome — https://doi.org/10.1053/j.gastro.2013.09.046
• [00:14] Fructan, Rather Than Gluten, Induces Symptoms in Patients With Self-Reported Non-Celiac Gluten Sensitivity — https://doi.org/10.1053/j.gastro.2017.10.040
• [00:16] Revised Estimates for the Number of Human and Bacteria Cells in the Body — https://doi.org/10.1371/journal.pbio.1002533
• [00:16] Microbial ecology: human gut microbes associated with obesity — https://doi.org/10.1038/4441022a
• [00:17] Altered gut microbiome composition in children with refractory epilepsy after ketogenic diet — https://doi.org/10.1016/j.eplepsyres.2018.06.015
• [00:18] An obesity-associated gut microbiome with increased capacity for energy harvest — https://doi.org/10.1038/nature05414
• [00:21] Dietary trehalose enhances virulence of epidemic Clostridium difficile — https://doi.org/10.1038/nature25178